1. Field of the Invention
The present invention relates to a fluorescent water base ink for ink-jet recording which has a high fluorescence intensity and in which the fluorescence intensity is not decreased as the time elapses.
2. Description of the Related Art
The ink-jet recording method is a recording method in which the recording is performed, for example, on the paper, the cloth, or the film by discharging the ink, for example, from the nozzle, the slit, or the porous film. Those known as the method for discharging the ink include, for example, the electrostatic attraction method in which the ink is discharged by utilizing the electrostatic attracting force, the drop-on-demand method in which the mechanical vibration or the displacement is applied to the ink by using a piezoelectric element, and the thermal ink-jet method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. The recording is performed by forming ink droplets with any one of the ink discharge methods as described above, and by adhering a part or all of the ink droplets to a recording objective member.
Researches for inks, which are applicable to a variety of fields by utilizing the ink-jet recording method as described above, are progressively advanced. In particular, the fluorescent ink, which contains a fluorescent dye, has such a property that the light (exciting light) having a specified wavelength is absorbed, and the light (fluorescence) having a wavelength longer than that of the exciting light is emitted. Therefore, for example, it is tried to apply the fluorescent ink to such a way of use that information, for which any security is required, is recorded on a recording medium, and the information can be read by generating the fluorescence by radiating the ultraviolet light as the exciting light. Further, for example, Japanese Patent Application Laid-open No. 9-291246 corresponding to U.S. Pat. No. 5,681,381 discloses a water-resistant water base fluorescent ink for imprinting postal indicia.
However, the following problem has been pointed out. That is, when the printing is performed on the paper with the fluorescent water base ink containing the fluorescent dye, the fluorescence intensity is lowered after several days, although the high fluorescence intensity is obtained immediately after the printing.